Existing equipment for testing electrochemical electrical energy generating and storage devices, such as fuel cells and batteries, are generally not fully optimized for such testing. This is quite apparent in several areas. The first example of such an area is the resistive load used to controllably consume the electrical energy produced or stored.
The simplest system, now rarely used, is a network of switches and resistors. This type of system is shown schematically in FIG. 1. Each resistor R is in series with a switch 10, and each resistor-switch pair is in parallel with all of the others. The parallel combination is placed across a power source, such as a fuel cell 12. As each switch is closed, the total resistance of the network drops. The resistors may be identical in size, or may vary to allow finer control of the total resistance.
One drawback of this system lies in the large number of mechanical contacts of the circuit as a result of the switches. Each mechanical contact adds a varied and changing resistance to the network. In a large network, consisting of a dozen or more switches, the network resistance will never be exactly the same twice. The unavoidable mechanical wear that occurs during use of the switches only adds to the problem by changing the resistance of different switches differently. One solution to this problem is the addition of a variable potentiometer in parallel with the switches, but this offers little improvement. Potentiometers suffer a similar drift in resistance characteristics with wear and age.
The next step in sophistication includes replacing the switches and resistors with field effect transistors (FETs). An FET acts like a resistor whose resistance can be varied over a wide range through the application of a bias voltage to one terminal of the device. In its simplest form a FET-based electronic load, which generally consists of a set of FETs mounted in parallel, is controlled by manually adjusting the gate voltage to produce the desired current flow through the system. This method is a distinct improvement on a resistor/switch network. It eliminates the contact problems present in the switches and gives virtually infinite variability in resistance over the entire available load range. While it is an improvement, this system is still inadequate. It requires manual operation to change the output of the system including periodic readjustments to offset variations produced by thermal effects in the circuit or variations in the cell's performance.
The FET-based circuit can be stabilized by the addition of analog circuitry tuned to measure and compensate for external changes. This circuit can be sufficiently complex to hold a constant current through changes in cell performance and to provide for external control of the load via an analog voltage supplied from an external source. A load of this type is sufficiently sophisticated to permit automation of the test system. The largest problem with this type of load circuit is its complexity. The large number of components involved increases the difficulty in fabrication and increases the chance of at least one component malfunctioning. The difficulty in fabrication and the number of components both contribute to making this an expensive system to produce and difficult to maintain.
Commercially available electronic loads, such as those produced by Hewlett Packard, have been used as loads for fuel cell and battery testing. While they are a distinct improvement on the loads described above, they are still deficient in several respects. They typically have maximum current capabilities on the order of 120 amps. While this is adequate for the uses they were designed for, it is insufficient for a high power density fuel cell. A 50 cm.sup.2 fuel cell operating at 3 A/cm.sup.2 puts out 150 amps, and some systems are routinely operated at up to 4 A/cm.sup.2, while many developers use cells with large areas. These units also have a second deficiency when used to test either single cell fuel cells or batteries. Since they were not designed for fuel cell testing or single cell battery testing, they were designed to operate with a typical minimum potential of 3 volts. If operated at lower voltages, the maximum current capacity is substantially reduced. Most fuel cells reach their maximum power output at around 0.6 volts, and at this voltage a 120 amp load's capacity is reduced to about 40 amps. This can be overcome by placing a power supply in series with the fuel cell or battery being tested to boost the voltage. While this works, there is some risk involved. For instance, if the gas supply to a fuel cell is cut off, the cell voltage can go to zero. Since the power supply is still applying a voltage, it is possible for the cell to be forced into reverse and become an electrolyzer. In this mode, the cell will generate hydrogen in the compartment that had previously contained oxygen, and oxygen in the one that had contained hydrogen. It is readily apparent that an explosive mixture could quickly be formed in either compartment. This is a situation that is to be avoided.
Still another type of electronic load can be constructed using a high speed chopper to turn the current through a fixed resistor on and off at high frequency (&gt;1000 Hz). When the chopper is on, the current flows at the same level as if the resistor were connected directly to the fuel cell. When the chopper is off, the current flow is zero. The duty cycle of the chopper is adjusted so that the total coulombic flux each second is the same as it would be through a fixed resistance larger than that of the actual resistor. One problem with this approach is that a fuel cell responds quite quickly to changes in resistance and tries to track the load. This can bias the results obtained in two ways. One of these is that a fuel cell's performance in a cycling system is not always the same as in a steady state system, even if the two systems have the same time averaged performance. Many types of batteries also show better performance under an intermittent load than a steady one and produce an overly optimistic result when tested under these conditions. The other bias is not in the cell, but in the measurement of the cell voltage under load with a fluctuating load. If the voltmeter being used doesn't average the voltage correctly at the chopper frequency being used, the results will be incorrect. For example, a DC voltmeter used to measure a high frequency AC voltage will give a reading greater than the true root mean square (RMS) value, and this will lead to overly optimistic, inaccurate results.
Humidification of fuel cell reactants is a second area where a variety of methods have been tried previously. While some have worked quite well, they generally have limitations that the present invention addresses.
The simplest way to humidify a reactant gas stream is to pass the gas as a stream of fine bubbles through water. As long as the gas has sufficient contact time with the water, controlling the temperature of the water controls the amount of water in the gas stream. This method works quite well at low gas flows, but as the required gas flow increases problems begin to arise. To fully saturate the gas with water requires either small bubbles, ideally under 0.5 mm in diameter, or a tall column to allow enough contact time to insure complete saturation. Operating the humidifier under conditions where the gas does not have sufficient contact time to become fully saturated and as a result is carrying a varying amount of water leads to an unstable situation.
While the cell itself is not likely to be damaged, the measurements made under these conditions may not be reproducible. For example, if a contact time of 0.5 seconds is required to saturate the bubbles with water, the column will need to be at least 19 cm tall (based on Stokes law velocity of 38.2 cm/sec for a 0.5 mm bubble of air in water at 80.degree. C.). For a flow rate of one liter of gas per minute, as 0.5 mm bubbles with an average spacing of 0.5 mm, a water volume of over 300 cm.sup.3 is required, with a similar or greater volume for the reverse portion of the convective flow produced by the gas lifting the water. Additional volume is required for the disperser to form the bubbles and for a reserve of water to replenish that lost to evaporation. The resulting humidifier has a volume of over one liter, and any increase in gas flow will require an even larger volume.
Another method that has been used is to humidify the gas inside the cell assembly, or stack, itself. This is usually done with a membrane humidifier. In this type of humidifier, a stream of water is located on one side of a water permeable membrane and the gas steam flows on the other side. This membrane may or may not be the same material as is used for the fuel cell. This method uses the heat of the cell itself to evaporate the water. This is both an advantage and a disadvantage. It eliminates the need for separate heaters to humidify the gas streams, but it limits the humidification of the gas streams to a dew point that is essentially the same as the cell's operating temperature. It also adds to the size of the cell stack. Since the humidifier is a structural part of the stack, it has to be built to serve as a supporting member. This can increase the weight and size of the system by a greater amount than is required for an external humidification system. Extra weight is always a disadvantage. This is the type of humidifier that is used in the well-known Ballard fuel cell system.
Still another humidification method is to inject water directly into either the manifold of the cell or stack, or a gas line leading into the manifold. The water is injected in such a manner as to form a mist in the gas line. As the gas stream is heated by the cell the water, which has a high surface area due to its small droplet size, quickly evaporates. This type of humidifier produces a very compact humidification system. The amount of water in the gas stream can easily be controlled by metering the liquid water feed into the cell. While this is a good system for stacks in the kilowatt range and larger, it is not an effective system for smaller systems. The weak point is the difficulty encountered in forming a steady consistent mist at low water flow rates. For instance, a nominally 1 kW stack consisting of six cells, each at 0.6 V, operating at 85.degree. C. with both the fuel and air streams humidified, requires about 10.3 grams of water per minute to humidify its air stream, assuming a 2:1 air to current stoichiometry at 30 psig. This amount is easily meterable on a consistent basis. A smaller stack, generating 300 W at 70.degree. C. requires only 1.50 grams of water per minute under the same feed conditions. This amount can be metered, but the higher precision required to maintain a smooth flow at the lower feed rate results in the smaller stack actually requiring a more complex humidifier. In the case of a small single cell operating at 30W, and the same operating conditions as above, the feed rate drops to 0.150 grams of water per minute for the air stream, and even less for the fuel gas stream. At these rates maintaining a steady flow is extremely difficult. Using a mist type humidifier under these conditions makes controlling the humidifier the most difficult part of operating the test cell.
Thus, there remains a need for a stable test system for testing electrochemical devices, such as fuel cells and batteries. Fine control of the humidification process, as well as accurate and reproducible control of the other components of the system, are paramount. Like the other items discussed above, a variety of methods have been used to control such test systems. The simplest and most basic system is direct manual control. This involves a trained operator reading a set of individual displays (dial gauges, digital voltmeters, strip chart recorders, etc.) and making all of the adjustments by hand. In this type of system, flow control is usually handled with a needle valve rotameter assembly. This method has poor precision, with the error in the measurement increasing with the flow rate. Although the flow is stable while both the back pressure and the feed pressure remain stable, changes in either pressure leads to changes in the flow rate. This can require frequent readjustment. Actual flow settings are determined with either a precalculated lookup table or recalculation for each adjustment. Load bank control is achieved by the operator manually adjusting the resistance as needed to maintain a stable current through the current measuring device. Since the load is held at a fixed resistance between resettings, any change in the cell's voltage is instantly reflected in a change in the cell's amperage. Needless to say, operating such a unit for an extended period requires a constant effort. Extended operation in the absence of an operator requires the addition of some type of safety shutoff. Since such a shutoff is perforce a simple one, it produces an immediate shutdown of the entire system which can cause damage to delicate components, such as the membrane of the humidifier. Run time and run continuity are lost in such a situation, even if the actual problem is a minor one.
Some of these problems can be eliminated through the use of electronic controls in place of the manual ones. For instance, replacing the needle valve rotameter assemblies with electronic mass flow controllers provides flow control that is independent of changes in gas pressure over a wide range. Settings for the mass flow controllers are obtained as described above. If an electronic load is being used, a feedback circuit can be used to maintain a steady current once the current has been set.
Some test stand designs use a separate computer to control all or part of the system. This is a great improvement on either of the control schemes described above. The combination of the computer with the mass flow controllers allows correct calculation of the gas requirements as the gas is needed.
Thus, there remains a need for a comprehensive test apparatus and method for control and testing of electrochemical generation and storage devices. If such a device is a fuel cell, the system must include a finely controlled humidifier that operates over a wide range of gas flow rates. The apparatus and method must also include means for automatically controlling an electronic load to accommodate variations in cell performance. Such a system therefore requires a plurality of data sensing and control functions.